The plans were a quest reward from Stratholme. Cataclysm saw the removal the quest. It is still possible to obtain the plans if a player held on to the item instead of using it, but chances of finding it on the Auction House are extremely slim. Blacksmiths who learned the plans before Cataclysm are still able to craft it. For more information on the quest, visit here.
"Post constructively or I'll press some different buttons next time." -- CM Daeleht (from the old WoW website, no source link)